The present invention relates to a photonic crystal fiber, and more particularly, to a novel method of fabricating a photonic crystal fiber, having a non-porous, all glass structure.
Communication systems which utilize optical fibers are known. These fibers typically achieve guiding of light by means of total internal reflection, based on the presence of a solid core of a relatively high refractive index that is surrounded by a solid cladding that has a relatively low refractive index.
A new type of optical fiber has recently been proposed which is referred to as a xe2x80x9cphotonic crystalxe2x80x9d or xe2x80x9cphotonic band gapxe2x80x9d (PBG) fiber. The PBG fibers involve a structure having a refractive index that varies periodically in space (in the X-Y plane). This type of optical fiber is discussed in several articles including J. C. Knight et al., Optics Letters, Vol. 21, No. 19, P. 15-47 (October 1996); T. A. Burkes, et al., Optics Letters, Vol. 22, No. 13, P. 961 (July 1997). These PBG fibers are typically fabricated with silica fiber having air gaps in order to achieve a periodic structure in the array which has a large index difference. This is achieved by the air gaps in combination with the silica fiber creating a lower refractive index in comparison to the areas having silica fiber alone. The air gaps are typically created by a multiple stack and draw process in which the air gaps are formed by holes drilled in silica rod preforms which are then stacked and drawn in order to create the PBG fiber structure.
A PBG fiber is also known in which that it was discovered that there was no need for a periodicty in the X-Y plane (cross-section) of the fiber. It was found that if the fiber possesses a core region having a refractive index that is significantly higher than the effective index of a fraction of a cladding region that surrounds the core region which comprises the multiplicity of micro structural cladding features such as capillary voids, that a periodic array was not necessarily required. However, capillary voids are still utilized as the primary means of forming the cladding material. However, the voids may be filled with metal or glass with a lower melting temperature than the capillary tube material in a subsequent operation with a second melt at a lower temperature. This introduces additional manufacturing time and costs, and also raises additional quality control issues.
The prior art process of making PBG fibers is difficult and costly, and it would be desirable to have simpler, less costly methods for making PBG fibers. Furthermore, these porous fibers are problematic for use in systems where it is necessary to have a solid or vacuum tight connection. It is also difficult to achieve a small bend radius with porous PBG fibers without damaging the fibers.
Briefly stated, the present invention provides a method of producing an all glass, non-porous, multi-component photonic band-gap fiber which includes the steps of creating a preform having a plurality of low refractive index glass rods and a plurality of high refractive index glass rods arranged in a pre-determined pattern between the low refractive index glass rods. The preform is heated and drawn to form a non-porous photonic band-gap fiber.
In another aspect, the invention provides for the assembly of the preform from a plurality of preform subassemblies which each have a predetermined number of low refractive index and high refractive index glass rods arranged in a predetermined pattern.
In another aspect, a method producing an all glass, non-porous, multi-component photonic band-gap multiple array is provided. The method includes creating a first PBG fiber by assembling a first preform having a plurality of low refractive index glass rods and a plurality of high refractive index glass rods which are arranged in a predetermined pattern between the low refractive index glass rods. The first preform is heated and drawn to form a first drawn non-porous subassembly having a first index. A second PBG fiber is created by assembling a second preform having a plurality of low refractive index glass rods and a plurality of medium refractive index glass rods which are arranged in a predetermined pattern between the low index glass rods. The second preform is heated and drawn to form a second drawn non-porous subassembly having a second index. A third preform is assembled from the first and second drawn non-porous subassemblies. The third preform is heated and drawn to form a non-porous multi-component PBG multiple index array.